Method and composition for etching titanium



United States Patent 3,082,137 METHOD AND COMPOSITION FOR ETCHING TITANIUM Mitchell A. La Boda, East Detroit, and Charles R. Wiese,

Detroit, Mich, assignors to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Filed Dec. 3, 1958, Ser. No. 777,819 3 Claims. (Cl. 15618) This invention relates to the shaping of metal parts and more particularly to the method of shaping metal parts by chemical dissolution techniques.

In recent years it has increasingly become more apparent that conventional methods of machining parts, particularly aircraft parts, have become inadequate for obtaining present day needs. Various methods of fabricating aircraft parts, for example, have been employed in the past to obtain high strength, exceedingly light weight articles. permit the manufacture of parts having the most desir able strength-'to-Weight ratios. This limitation in conventional manufacturing techniques is particularly important where a conventional, relatively heavy material must be used in aircraft or rocket parts. When making turbine or compressor wheels of a jet engine, for example, the web portion of the wheel can be quite thin. Conventional manufacturing methods cannot be used under commercial production conditions to satisfactorily form the webbing to the most desirable thinness.

Moreover, in the manufacture of metal parts, frequently the parts must be machined after being formed by casting, forging or the like. When the part is machined a burr ordinarily results at the edge of the surface which is machined. The presence of a burr on a finished part is ordinarily objectionable and must be removed. Conventionally burr removal is accomplished by an abrasive blast, tumbling, shot peening, etc. However, conventional methods are quite costly and time consuming.

By utilizing chemical dissolution technique parts made of titanium or aluminum have been manufactured so as to have optimum strength-to-weight ratios. These tech niques, used in conjunction with conventional machining operations, permit the formation of satisfactory, exceedingly thin formed metal sections economically under production conditions. Moreover, the use of chemical dissolution techniques further provides an economical means whereby burrs on machined parts can be removed.

It is universally recognized that the surface finish of a part which is subjected to physical and thermal stresses should be exceptionally smooth. It is accepted that extremely smooth surface finishes are not only desirable but, in most instances, necessary to obtain the optimum resistance to fatigue and corrosion. Titanium and titanium alloys have been chemically machined by acid dissolution in which there is a simultaneous hydrogen generation. The hydrogen generated is believed to be absorbed in the titanium metal, which action seriously impairs the fatigue resistance of the metal through hydrogen embrittlement. Moreover, a solution may be available which will dissolve titanium, for example, but in so doing will produce the smooth surface finish required. In making a part utilizing chemical dissolution techniques the primary problem is not to merely obtain metal removal but to obtain it in a controlled manner. Moreover, in some instances,

However, conventional methods still do not "ice it is even a problem to obtain a solution which will dissolve a given metal at a satisfactory rate.

The satisfactory dissolving of a given alloy requires a highly specific solution which will dissolve the metal and simultaneously either give rise to a smoother surface finish, or at least maintain the existing surface finish of the part. It has been found that, in general, solutions which will dissolve an alloy will simultaneously produce a rough, etched, striated or pitted surface. The solutioning of a given metal must, therefore, be done under controlled conditions, in a specific manner, with a particular type of solution to obtain the surface finish which is desired. By means of this invention titanium or a titanium base alloy, which is of extreme importance in the manufacture of aircrafts and missiles, can be satisfactorily machined by chemical dissolution techniques.

It has now been found that titanium or a titanium base alloy containing at least about 85% titanium can now be satisfactorily machined by chemical dissolution in an aqueous bath containing potassium permanganate and hydrofluoric acid. In this bath metal is removed from titanium base alloys at a 'rapid rate without any deleterioussurface'effects.

For example, the results obtained withthe titanium base alloy, commonly referred to as AT, are especially satisfactory when treated in accordance with our invention. The alloy ,11() AT has a general composition.

which is as follows:

. Percent Aluminum 5 Tin 2.5 Titanium Balance By means of this invention parts made of titanium base alloys, such'as 110 AT, can be chemically readily machined to exceptionally thin sections having an exceptionally smooth surface finish. This invention further provides the method of chemically deburring parts formed of such alloys in a satisfactory economical manner. Now titanium base metals can be satisfactorily machined under commercial production conditions by chemical dissolu tion techniques.

'In order to form a metal part by means of chemical dissolution techniques, the part is initially cast or premachined to desired measurements prior to the chemical dissolution treatment. It is generally desirable to form the part slightly oversize when no masking is employed so that in the chemical dissolution treatment the part is reduced to finish dimensions. It is preferred to form the part to as close to final desired specifications as is practical by conventional techniques since metal removal by chemical dissolution is somewhat more time consuming. The performed part can then be placed on a suitable support means where it is cleaned.

Although various methods of cleaning may be employed, highly satisfactory results have been found to be obtainable when the part is initially degreased in a trichloroethylene vapor at a temperature of approximately F. However, one of the many commercially available di-phase cleaners which are stable emulsions of an organic cleaner and an alkali cleaner, can be used.

After degreasing, the part is dried and then subsequently immersed in an alkaline solution and electrically cleaned. A solution containing soda ash 35%, trisodium phosphate 55% and caustic soda 10%, all proportions 3 by weight, can be used. In general, it is desirable to include a wetting agent, such as sodium resinate, in the bath in quantities up to 5%, by weight.

The part is subjected to an anodic potential of about six volts for approximately one minute and subsequently rinsed with water. The duration of the anodic cleaning is variable, depending upon the size, configuration, etc. of the part and upon the freshness of the cleaner. The effectiveness of the cleaner decreases in use so that more extended periods of anodic cleaning may be required for a part in a cleaning solution that has been used a number of times.

.After rinsing the part free of any of the cleaning solu tion that may adhere to its surface, it is dried. The part is then ready to be chemically machined. The part is preferably positioned in the machining solution in such a manner as to avoid non-uniform dissolution of the surfaces. Gas generated during the dissolution of the metal part can accumulate in recesses or horizontal areas of the part so as to interfere with uniform chemical dissolution of the entire surface. When chemical machining an article having a planar configuration, such as a panel, it is desirable to support the panel in the machining solution in a vertical attitude.

On the other hand, articles of a more complicated configuration containing complex contours and recesses may not be suitably maintained in any position which will entirely inhibit collection of the generated gases and formation of gas pockets. For these and other types of articles itmay be desirable to chemically machine the parts in a plurality of steps in which portions of the part are masked from the solution. For example, the top of such an article can be chemically machined while its lower surface is masked with a suitable stop-01f material. When sufficient metal removal of the upper surface is obtained, the-part is removed from the solution, rinsed, and the stop-ofi' removedto expose the protected surface. The machined'surface is then masked and the part reimmersed for completion of the chemical machining of the part. Of course, to complete the machining, the part is inverted so that the masked surface is on the bottom of the part.

Titanium base metals, such as pure titanium and titanium base alloys containing at least 85%, by weight, titanium, particularly titanium base alloys formed of, by weight, about 1% to aluminum, about 1% to 5% tin and about 85% to 98% titanium can be chemically machined in an aqueous solution which is made up approximately as follows:

Potassium permanganate "grams" 25 to 75 Concentrated hydrofluoric acid (52% to 55%) milliliters 30 to 90 Water do 1500 The chemical machining of a titanium base alloy containing aluminum, tin and at least about 85% titanium, by weight, can be satisfactorily solution machined in a bathsolution of the above composition. However, in some instances, a titanium base metal containing at least 85% by weight, titanium and particularly these metals containing higher amounts of titanium, can be satisfactorily solution machined when the concentrated hydrofluoric acid (52% to 55%) in the bath is as low as about 25 milliliters and as high as approximately 150 milliliters.

More specifically, we have found that a highly satisfactory. surface leveling effect is produced when a part composed of 110 AT titanium alloy is immersed in a suitable bath solution at a temperature of about 150 F. We have found highly satisfactory results to be available when dissolving 110 AT in a solution which is formed as follows:

Although optimum results with the above solutions are obtained when the metal removal is accomplished at a bath temperature of approximately 150 F., it has been found that highly satisfactory results are obtained employing bath temperatures of approximately 100 F. to 200 F. At these bath temperatures not only is original surface smoothness retained but a highly desirable surface leveling is additionally eflected.

If stopping-off of the part to be machined is required, generally any maskant can be used which is insoluble in the bath solutions described-- above. However, stop-off materials such as a phenolic base lacquer or a polyvinyl base lacquer can be used. Of course, when employing phenolic base lacquers the masked part is subjected to a temperature of approximately 350 F. for several minutes to bake the lacquer. The usual time for air drying of polyvinyl base lacquers can be reduced by heating for several minutes at temperatures up to approximately 250 F.

After the stop-off has been applied, the part is again cleaned to remove any fingerprints or any adhesive from some of the stop-off materials which might have been used. Generally, fingerprints can be removed by wiping the surface of the part with'a suitable solvent. One such solvent which can be used with a phenolic base lacquer is methyl ethyl ketone. When using a polyvinyl base lacquer, however, a more satisfactory solvent would be methyl alcohol or some other solvent which. does not dissolve the base lacquer as does methyl ethyl ketone. In some instances it may beadditionally desirable to wipe the surface with a second solvent whichis more particularly suited to the removal of the adhesive which is used portant in forming an improved part. It is believed that,

with increases in surface'smoothness, the fatigue life of a part is not only increased but'that its corrosion resistance is also materially increased. Our invention provides a means whereby such surface smoothness can be readilyobtained on an alloy of a specific composition. The rate of chemical dissolution between the grains, grain boundaries, inclusions, etc. of a metal are not generally equal. A highly specific solution used under specific conditions must be employed in order to obtain a uniform rate of dissolution of the various phases of the metal composition. Our invention provides for rapid metal removal without deleterious surface activity.

Although this invention has been described in connection with certain specific examples thereof, no limitation is intended thereby except as defined in the appended claims.

We claim:

1. An aqueous bath solution for the chemical machining of a titanium base metal, said bath solution consisting essentially of, in addition to water, about 50 grams potassium permanganate and about 60 milliliters of concentrated hydrofluoric acid, respectively, for each 1500 milliliters of water in the solution.

2. A method of chemically machininga metal part which comprises applying an aqueous solution consistingessentially of, in addition to water, about 50 grams potassium permanganate and about 60 milliliters concentrated hydrofluoric acid, respectively, for each 1500 milliliters of water in the solution to asurface of a part made of a titanium basemetal to chemically remove metal from said surface to form a predetermined configuration.

3. The method of chemically machining a metal part which comprises immersing a part made of a titanium base alloy having at least about by weight, titanium in an aqueous solution consisting essentially of, in addition to water, about 50 grams potassium permanganate and 60 milliliters of concentrated hydrofluoric acid, respecin said solution for a suificient duration to dissolve metal from a surface of the part to form a predetermined oon- 5 figuration.

References Cited in the file of this patent UNITED STATES PATENTS MacPherson Nov. 22, 1955 1 6 Landgren Aug. 12, 1958 Topelian Aug. 12, 1958 Otto Oct. 14, 1958 Bomberger et a1. Mar. 3, 1959 McCord et al. Jan. 19, 1960 OTHER REFERENCES Metals Handbook, 1954 supplement, April 1954, published by American Society for Metals, page 84, co]. 1, 0 paragraph 1 and Table V.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N06 3,082,137 March 19, 1963 Mitchell A, La Boda It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 39, for "technique" read techniques llne 60. after "will" insert not Signed and sealed this 1st day of October 1963.

(SEAL) Attest:

ERNEST Wt SWIDER DAVID L LADD Attesting Officer Commissioner of Patents 

2. A METHOD OF CHEMICALLY MACHINING A METAL PART WHICH COMPRISES APPLYING AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF, IN ADDITION TO WATER, ABOUT 50 GRAMS POTASSIUM PERMANGANATE AND ABOUT 60 MILLILITERS CONCENTRATED HYDROFLUORIC ACID, RESPECTIVELY, FOR EACH 1500 MILLILITERS OF WATER IN THE SOLUTION TO A SURFACE OF A PART MADE OF TITANIUM BASE METAL TO CHEMICAL REMOVE METAL FROM SAID SURFACE TO FORM A PREDETERMINED CONFIGURATION. 